1. Field of Application
The present invention relates to an exposure control apparatus, for exposure control of an electronic camera which captures successive images of a region ahead of a vehicle in which the camera is installed.
2. Description of Related Art
In recent years, vehicle-installed electronic cameras (in general, digital video cameras, referred to in the following simply as cameras) have come into use for capturing images of a region located ahead of the vehicle, with technology having been developed whereby the captured images (sets of digital data expressing respective captured images) are processed for such functions as recognition of white lines formed on the road surface or detection of objects such as a preceding vehicle. The processing results can be used to generate warning indications to a vehicle driver, control driving of the vehicle, etc.
With such technology, it is important that the exposure of the camera be appropriately controlled in accordance with brightness of the scene which is being captured by the camera, in order to maximize the reliability of recognizing white lines on the road surface or solid objects such as preceding vehicles, etc., based on the images captured by the camera.
A vehicle having such a camera and processing apparatus installed therein, which are being described, is referred to in the following as the “local vehicle”.
As described for example in Japanese patent first publication No. 6-25320B (designated as reference document 1 herein), a method of using such camera images for recognition of white lines on the road surface has been proposed whereby two laterally extending sections are selected within each image. A first one of these sections is positioned to contain a part of the (imaged) road surface that is currently close to the local vehicle, and the data obtained from the first section, in each of successive captured images, are utilized for recognition of white lines on the road surface. The second section is positioned to contain a part of the road surface that is farther ahead of the local vehicle (i.e., is in an upper part of each captured image). Hence the second section contains a region which will be subjected to recognition processing at a future time point, determined by the speed at which the local vehicle is travelling. Designating the average brightness levels of the first and second sections as b0 and b1 respectively, the difference between these is obtained for each of successive captured images. If the difference is found to exceed a predetermined threshold value, then the camera exposure which will be applied in capturing the next image is adjusted based on the brightness value b1 (i.e., by changing the camera shutter speed, etc).
With the above method of reference document 1, if for example the road surface ahead of the vehicle changes between a brightly sunlit condition and a shade condition, the camera exposure can be appropriately controlled for each of successive captured images, i.e., such as to prevent the abrupt change in scene brightness from affecting the reliability of white line detection.
However in an actual road environment, the brightness of the road surface will not generally change between a sunlit condition and a shade condition (i.e., with the change extending uniformly across the road surface) in such a simple manner. Instead, the changes can take various forms. For that reason, it is difficult to reliably control the camera exposure by such a method under actual operating conditions.
It has also been proposed, for example in Japanese patent first publication No. 2005-148308 (designated as reference document 2 herein) to use an exposure control apparatus whereby the brightness of the road surface ahead of a local vehicle is measured for use in camera exposure control, while excluding the effects of white lines (traffic lane markers) formed on the road surface. A video camera on the vehicle obtains successive captured images of a region directly ahead of the vehicle, which contains these white lines. A plurality of areas within each captured image are selectively examined to measure their respective brightness levels, with these areas being predetermined as corresponding to areas of the road surface that are normally outside the white lines when the vehicle is travelling along the center of a traffic lane. The respective brightness values of these areas are measured, and the exposure of the vehicle-mounted camera is controlled based on the results.
Diagram (a) in FIG. 17 shows an example of such brightness measurement regions, i.e., areas used for measurement of the brightness level of the road surface. Such a method is intended to enable the brightness of the road surface to be more reliably measured, without the measurement being affected by fluctuations in brightness due to presence of white line portions in the captured images. However as illustrated in diagram (b) of FIG. 17, if the local vehicle moves laterally, then portions of the white lines may come within the brightness measurement regions. This results in fluctuations in the capture-objective brightness values that are derived from successive captured images. Due to these fluctuations, appropriate exposure control may not be possible.
If it is attempted to set the positions and configurations of the brightness measurement regions such that the white lines will never come within them, then the freedom of determining the locations and shapes of these brightness measurement regions becomes severely limited. Hence, due to such factors as variations in regulations concerning vehicle width, positions of white lines used to define traffic lanes, etc., in various different countries, it may be difficult to establish appropriate brightness measurement regions, so that an excessive amount of work is required in design, etc., with such a method.